1. Field of the Invention
The invention relates to a method of manufacturing a semiconductor device having an electroless plating electrode as a draw-out electrode, particularly a method of manufacturing a semiconductor device having a trimming element including a conductive fuse.
2. Description of the Related Art
There is a case in which a conductive fuse 55 as a trimming element is formed on a semiconductor substrate 51 as well as a device element so as to adjust the circuit characteristics etc of the element portion. FIG. 5A shows an enlarged plan view of an ordinary trimming element including a conductive fuse 55, and FIG. 5B shows a cross-sectional view of FIG. 5A along line C-C. Some of the reference numerals in FIGS. 5A and 5B will be described below referring to FIGS. 6A and 6B.
As a method of trimming by blowing out a conductive fuse 55, there are an electric current blowout trimming method in which the conductive fuse 55 is blown out by a high electric current and a laser trimming method in which the conductive fuse 55 is blown out by laser light. In either of the methods, as shown in FIG. 5B, a silicon nitride (SiN) film 59 for passivation covering the topmost surface of a semiconductor substrate 51 is removed from on the conductive fuse 55 as a trimming element by etching.
This is to avoid the following problems. In the case of the laser trimming method, when the conductive fuse 55 is covered with the SiN film 59 for passivation, the applied laser light is absorbed in the SiN film 59 to attenuate the laser light to reach the conductive fuse 55 and decrease the trimming efficiency. In the trimming method of the electric current blowout type, an escape route for the residues of the blown out conductive fuse 55 is closed, thereby leading to the reconnection of the blown out conductive fuse 55.
In this case, moisture etc that enters a trimming element forming region 70 where the SiN passivation film 59 is removed by etching may enter a region 71 where a device element such as a MOS type transistor is formed through interlayer insulation films 56 and 54 as shown by an arrow 62 in FIG. 5B and may change the characteristics of the device element, causing a problem in the reliability. The trimming element forming region 70 having a small occupation area is surrounded by the device forming region 71 having a large area.
In the trimming element forming region 70 where the conductive fuse 55 is blown out by trimming, the conductive fuse 55, the interlayer insulation films 56 and 54, etc are partially broken and allow moisture etc to enter especially easily. This is because moisture etc enters from the sidewalls of the interlayer insulation films 56 and 54 in the broken portion or the end of the broken portion extends to inside the interlayer insulation film 54 close to the semiconductor substrate 51.
A method to address this problem is disclosed in Japanese Patent Application Publication No. Hei 05-063091. FIG. 6A is an enlarged plan view of a trimming element forming region 70 and its vicinity, showing the disclosed content. In detail, the trimming element forming region 70 including a conductive fuse 55 of which the center portion is narrowed for easy blowout as a fuse blowout portion 55a is surrounded by a guard ring 58 of a metal layer, so that moisture etc entering the trimming element forming region 70, which is shown by an arrow 62, is prevented from entering the device forming region 71 as shown in FIG. 6B. The conductive fuse 55 is drawn out to the device forming region 71 surrounding the guard ring 58 through the opening 60 formed in the guard ring 58.
FIG. 6B is a cross-sectional view of FIG. 6A along line C-C. A SiN film 59 for passivation omitted in FIG. 6A is also shown. An N type epitaxial layer 52 is formed on a P type semiconductor substrate 51, and a P+ type isolation layer 53 divides the epitaxial layer 52 in the trimming element forming region 70 and an epitaxial layer 52a in the device element forming region 71.
A field oxide film 54 is formed on the P type semiconductor substrate 51 including the epitaxial layer 52 etc. Furthermore, a first metal wiring 58a is connected to the P+ type isolation layer 53 through a contact hole formed in the field oxide film 54. Furthermore, the conductive fuse 55 made of polysilicon is formed on the field oxide film 54. The fuse blowout portion 55a of the conductive fuse 55 is shown in FIG. 6B. The conductive fuse 55 is covered with an interlayer insulation film 56.
A through-hole is formed in the interlayer insulation film 56, and a second metal wiring 58b is formed so as to be connected to the first metal wiring 58a through the through-hole. The first metal wiring 58a and the second metal wiring 58b unite to form the guard ring 58. A passivation insulation film 57 having an opening is formed on the interlayer insulation film 56 etc, and the SiN passivation film 59 is further formed so as to cover the passivation insulation film 57 and have an opening on the conductive fuse 55.
As a result, as shown in FIG. 6B, moisture etc entering the trimming element forming region 70 from above the interlayer insulation film 56 on the fuse blowout portion 55a where the SiN passivation film 59 is removed by etching, as shown by an arrow 62, is blocked by the guard ring 58 of a metal layer connected to the P+ type isolation layer 53, thereby preventing the moisture etc from entering the device element forming region 71 outside the guard ring 58.
In Japanese Patent Application Publication No. Hei 05-063091, most of the trimming element including the fuse blowout portion 55a is surrounded by the guard ring 58. Therefore, even when moisture etc enters from the opening where the SiN passivation film 59 above the trimming element is removed by etching, as shown by the arrow 62 in FIG. 6B, the guard ring 58 prevents the moisture etc from entering the device forming region 71.
However, as shown in FIG. 6A, the opening 60 is formed in a portion of the guard ring 58, and the conductive fuse 55 is drawn out to the device element forming region 71 outside the guard ring 58 through the opening 60. In this case, in order to avoid a short circuit between the conductive fuse 55 and the guard ring 58 at the drawing portion, a narrow guard ring separation 60a is provided between both the side surfaces of the conductive fuse 55 and the guard ring 58.
Although narrow, this guard ring separation 60a becomes a path of moisture etc to the device element forming region 71 outside the guard ring 58, which enter from the opening in the trimming element forming region 70 where the SiN passivation film 59 is removed by etching. Although only one conductive fuse 55 is formed in FIG. 6A, there is also a case where a plurality of conductive fuses 55 are formed, and in such a case, the path of moisture to the device forming region is wider, thereby causing a problem in the reliability.
Therefore, it is necessary to realize a semiconductor device in which moisture etc does not enter the device forming region 71 from the opening in the trimming element forming region 70 where the SiN passivation film 59 is removed by etching. Particularly, when an electroless plating electrode is formed on an draw-out electrode that is the topmost layer wiring of multilayered wirings after the fuse blowout portion 55a is blown out by trimming, it is necessary to prevent the reconnection of the blown out fuse blowout portion 55a by a plating metal and prevent a plating solution etc from entering the complexly broken trimming element forming region 70, which cause a problem in the reliability.